Modern methods for stimulating hydrocarbon flow in oil and gas wells has increased demand for well servicing knowledge and equipment. Common well servicing operations include removing and installing spools, valves, blowout preventors and other elements in a control stack on the wellhead, inserting downhole tools into the well, and/or injecting high-pressure well stimulation fluids. Modern equipment permits many of these operations to be performed without killing the well. This is advantageous because killing fluids are expensive and have the potential to reverse the beneficial effects of a well stimulation procedure.
The Applicant has invented many methods and tools for protecting wellhead components from pressurized fracturing fluid, to permit the fluid to be pumped into hydrocarbon wells, as described, for example, in co-pending United States patent application U.S. Pat. No. 6,364,024, entitled BLOWOUT PREVENTOR PROTECTOR AND METHOD OF USING SAME, which issued on Jan. 28, 2000. The fracturing fluids may be strongly acidic, or alkaline fluids and may be loaded with an abrasive proppant such as bauxite or sharp sand. Applicant's blowout preventor protectors have several advantages over the prior art, particularly because they permit a tubing string to be run into or out of the well, and accessed during the stimulation treatment. In order to insert the blowout preventor protector into the wellbore, the tubing hanger must be removed. Before the tubing hanger can be removed, the tubing string must be plugged to prevent an escape of hydrocarbons to atmosphere. This is preformed by setting a plug in the tubing string using a wireline lubricator, for example. This is an expensive and time consuming procedure that requires the use of wireline equipment to set the plug, as well as to remove it. As can be appreciated by those skilled in the art, if the tubing string is to be used during the stimulation process, for example as a “dead string” used to monitor downhole pressure, or as an extra stimulation fluid conduit or “flow back” tubing, the wireline plug must be set and removed two times during the well stimulation process. A first time to remove the tubing hanger, and a second time to re-attach it to the tubing string.
As is known in the art, some tubing hangers have backpressure threads for receiving commercially available plugs to seal the tubing string. Thus, the tubing hanger serves as a common point for sealing both annular and circular spaces in the well, as is well known in the art. Consequently, it is possible to remove and install the control stack elements without having to plug the production tubing using a wireline plug. However, if the tubing hanger is removed, the plug is removed with it, leaving the tubing string open to atmosphere. Since many well servicing operations require that the tubing hanger be removed and/or set, it is generally necessary to call in wireline equipment with crew at least twice during each such well servicing procedure.
To reduce the costs associated with well servicing procedures, it is therefore desirable to provide a method and apparatus for permitting a tubing string to be plugged below a tubing hanger without the use of wireline equipment.